The present invention concerns an ink jet ink, and in particular a water-based, flash-curable ink jet ink that is capable of adhering to difficult substrates such as matt or gloss uncoated vinyl surfaces.
Printed ink jet inks on substrates are currently cured by medium and high pressure mercury vapour UV lamps. One of the disadvantages of using these lamps is that they are switched on for long periods of time which causes a high degree of heat to be produced and makes it necessary to have sophisticated cooling equipment. The cooling equipment produces either a high air flow or water cooling. The cooling equipment is bulky and it is expensive to incorporate it into an ink jet printing machine. Failure to cool the UV lamp causes distortion of the substrate. The other disadvantages of using a standard UV lamp are: surfaces exposed to the UV lamp need to be equidistant from the lamp in order to achieve uniform curing; and the UV lamp can cause yellowing of the cured surface.
WO 94/11123 has overcome the disadvantages of a standard UV-curable lamp by proposing the use of a high energy electronic flash source and the use of photoinitiators that correspond to the radiation emitted from the flash source. WO 94/11123 discloses a flash curable protective coating that is printed on to optical articles made of plastic (see Example 6). The protective coating comprises 30-40 parts multifunctional melamine acrylate; 70-60 parts trifunctional reactive thinner; 0.5-1.5 parts p-phenyl benzophenone; 1.5-3 parts methyl methyl thiophenyl morpholinopropane; and 0.1-0.5 parts polyether modified dimethyl polysiloxane copolymers.
The inventor of the present invention coated flexible polyvinyl chloride with the protective coating disclosed in Example 6 of WO 94/11123 and found that the coating cracked badly when the polyvinyl chloride was folded through 180xc2x0. The protective coating disclosed in WO 94/11123 is therefore not suitable for printing on to difficult plastic substrates such as flexible polyvinyl chloride.
The present invention is concerned with the problem of providing a water-based, flash-curable ink jet ink that is capable of adhering to difficult plastic substrates such as flexible polyvinyl chloride laminates that are used in display signs.
In accordance with the present invention there is provided a water-based, flash-curable ink that is capable of adhering to difficult plastic substrates such as flexible polyvinyl chloride, the ink comprising:
a) at least one water dispersible or water soluble acrylate polymerisable material;
b) at least one polymerisable material that is capable of softening the substrate in part or in full and is resistant to hydrolysis;
c) a colourant such as a pigment, a dye or both a pigment and a dye; and
d) at least one UV photoinitiator.
The present inventors have found that the ink defined above is capable of adhering to difficult plastic substrates such as flexible polyvinyl chloride.
By the term xe2x80x98polymerisable materialxe2x80x99 we intend to include monomers, prepolymers and oligomers.
The water dispersible or water soluble acrylate polymerisable material is preferably a water dispersible or water soluble urethane acrylate oligomer.
The polymerisable material in part b) is preferably a monomer, and the monomer is preferably 1,6-hexane diol diacrylate (HDDA). HDDA is a difunctional acrylate that offers a high cure rate. HDDA also exhibits excellent compatibility with and solvency for other oligomers and additives used in the ink.
The water dispersible or water soluble acrylate polymerisable material may be non-ionic or anionic. The water dispersible or water soluble acrylate polymerisable material preferably has a molecular weight between 400 and 1,500, more preferably between 700 and 1,100, as measured by size exclusion chromatography. The non-volatile content of the polymerisable material may be between 10 and 100%.
The photoinitiator may comprise one or more photoinitiators and also a synergist. The photoinitiator preferably has a quantum yield in the range 0.1 to 1.0. The photoinitiator should not function as a chain transfer agent or terminating agent.
The preferred photoinitiators are Lucerin TPO acyl phosphine oxide (sold by BASF), Irgacure 369 (sold by Ciba-Geigy) and Darocure 1173 (sold by Ciba-Geigy). Darocure 1173 is a useful solvent for the Lucerin TPO and the Irgacure 369. The Lucerin TPO and Irgacure 369 are preferably used in amounts ranging from 0:1 to 1.0%, based on oligomer solids. The Irgacure is preferably used in an amount ranging from 0.1 to 3.0%, based on oligomer solids. 4-benzoyl biphenyl is also a useful photoinitiator.
The pigment may be dispersed in the ink using any one of the known methods that produces a fine particle size of less than 1 micron. The pigment may also be added to the ink in a dispersed form.
Examples of suitable pigments are: Yellow 13 and 83; Red 9, 184 and 188; Blue 15:3; Green 7; and Black 7. The reader is directed to U.S. Pat. No. 5,160,372 for further examples of suitable pigments.
The dye may be selected from known-dyes that are used in ink jet systems. The reader is directed to U.S. Pat. No. 5,160,372 for examples of suitable dyes.
The ink can include any of the usual additives that are present such as: surfactants, UV stabilisers, waxes and rheology modifiers.
The present invention will now be described by way of the following Examples: